Generally, the invention relates to an apparatus and method for manufacturing printed circuit boards and for assisting the process of soldering metals to integrated circuit boards. More specifically, the invention relates to a system for measuring and controlling the height of a solder wave that will provide more consistent soldering applications during the manufacturing process.
Generally speaking, in a wave soldering machine a printed circuit board (PCB) is moved by conveyor on an inclined path past a fluxing station, a preheating station, and, finally, a station at which a wave of solder is caused to well upwardly and contact various portions of the PCB to be soldered. The efficiency of this wave soldering process is affected by a number of concerns, two of which are known in the art as the xe2x80x9ccontact-areaxe2x80x9d and the xe2x80x9cdwell-time.xe2x80x9d The dwell-time represents the amount of time any given area of the PCB is in actual contact with the solder. Generally, the dwell-time is related to the contact-area and can be determined by simply dividing the length of the contact-area, which is in the direction of travel, by the speed of the conveyor belt.
The contact-area represents that portion of the PCB that is in actual contact with the solder wave, i.e., the area of the PCB that is currently covered by the solder wave. It is preferable to have a contact-area that is uniform, i.e., not uneven. A uniform contact-area is one where the dwell-time is the same for all portions of the PCB. For example, for a rectangular shape all portions of the PCB perpendicular to the direction of motion enter, and leave, the solder wave at approximately the same time. However, if the contact-area is not uniform, then some portions of the PCB are within the solder bath longer than other portions, i.e., the dwell-time changes across the PCB. For example, if the contact-area is trapezoidal (with the parallel sides in the direction of travel), one side of the PCB maintains contact with the solder longer than the opposite side. If the dwell-time is too short, the soldering process may not be completed. On the other hand if the dwell-time is too long, the PCB may become too soft and begin to sag, or solder may go up through the apertures of the PCB due to strong capillary action, which could result in solder bridges forming on the component side of the board.
One method for measuring the solder wave contact-area and the dwell-time is to use a tempered glass plate that is passed over the solder wave during a xe2x80x9ctest pass.xe2x80x9d This glass plate has a set of grid lines of known dimensions. As the glass plate passes over the solder wave, the contact-area is visually observed, and approximately measured via the grid lines, by an operator of the solder wave machine. From the observed contact-area, the dwell-time is determined by dividing the length of the contact-area, i.e., that dimension in the direction of motion of the conveyor belt, by the speed of the conveyor belt.
Unfortunately, this method for measuring the contact-area is purely subjective and only yields an approximation of the contact-area. In addition, in order to observe the contact-area on the glass plate, the wave soldering machine conveyor speed must be slowed so that the operator has time to observe the glass plate.
Other problems occur when the height of the solder wave that comes in contact with the PCB is not at an optimum level. If the wave height is too high, solder will flow on the top surface of the PCB and ruin the board or at least cause the PCB to fail. If the height of the solder wave is too low, some of the components may not be soldered appropriately and the PCB may fail or at least fail prematurely in time.
Employees are often reprimanded if they adjust the height of the wave to high whereby the solder flow gets onto the top surface of the PCB. Thus many employees, tend to adjust the wave height too low thereby giving inconsistent solder to the bottom of the PCB.
Therefore, a unsolved need exists for an apparatus and method for processing PCBs and for assisting the process of soldering metals to PCBs, which overcomes limitations and deficiencies of the prior art.
One object of the present invention is to provide an apparatus and method for processing printed circuit boards and for assisting the process of soldering metals to integrated circuit boards.
Another object of the present invention is to provide a system for measuring and controlling the height of a solder wave that will provide more consistent soldering applications during the printed circuit board manufacturing process.
In accordance with principles of the present invention, an apparatus for processing printed circuit board is set forth having a system for measuring and controlling the height of a solder wave which is generated by a solder bath associated with the printed circuit board manufacturing process. The apparatus for processing printed circuit boards includes a conveyor system for transporting printed circuit boards through a number of processing stations. The conveyor system includes a pair of elongated longitudinal conveyor rails which are oriented parallel with respect to each other. The conveyor further includes a plurality of pairs of printed circuit board gripping fingers which are mounted on a drive belt defined on each of the rails. Each pair of gripping fingers are oriented to face each other for enabling the gripping fingers to securely grip opposing edge regions of a printed circuit board for transporting the printed circuit board through a number of processing stages including the wave soldering process.
In an embodiment, the system for measuring and controlling the solder wave height defined in the wave soldering process includes a sensor which is mounted in close proximity to the top surface of the solder wave. The sensor is coupled to a micro-controller, via sensor electronics, and the micro-controller is coupled to a pump motor. The pump motor is coupled to the solder bath and generates the solder wave by controlling the pump motor to operate at a predetermined revolution-per-minute to maintain a predetermined solder wave height during the process of wave soldering printed circuit boards.
The sensor can be mounted in a preformed aperture formed on one of the rails, which is defined in close proximity to the top surface of the solder wave. In this orientation, the sensor can communicate with the top surface of the solder and generate an analog voltage sample representing the distance between the sensor and the top surface of the solder wave. The analog voltage sample is communicated to the micro-controller and is converted into a predetermined value, which is relative to the distance between the top surface of the solder and the bottom surface of the printed circuit board. The micro-controller includes a comparator, which receives the value and compares the value to a predetermined set point. If the value is too high, the micro-controller controls the pump motor to reduce the speed of the motor and if the value is too low, the micro-controller controls the pump motor to increase the speed of the motor.
In an embodiment, the micro-controller further includes a register. The register is adapted to receive a plurality of values. The values represent a plurality of distance samples which are each relative to a distance defined between the top surface of the solder wave and the bottom surface of the printed circuit board. The values contained in the register can be averaged to provide an average distance defined between the top surface of the solder wave and the bottom surface of the printed circuit board. The comparator receives the average value and compares the average value to the predetermined set point to determine whether the solder wave height is too high or to low and to respectively decreases or increases the speed of the pump motor similarly to that previously described.
A method of using the apparatus for processing printed circuit boards including the system for measuring and controlling the wave height of solder comprises the steps of turning on the apparatus for processing printed circuit boards and pausing momentarily for the pump motor to form a solder wave having a predetermined solder wave height; sampling a plurality of analog voltage samples; providing the analog voltage samples to a micro-controller for enabling the micro-controller to convert the analog voltage samples to a plurality of values; providing the values to a register; determining an average value from the plurality of values defined in the register; comparing the average value with a predetermined set point to determine if the average value is within a predetermined tolerance range of the set point, if the average value is within the predetermined tolerance range of the set point, then repeating the steps of determining an average value and comparing the average value with a predetermined set point, and if the average value is not within the predetermined tolerance range of the set point, then determining whether the average value is too high or too low; and controlling the pump motor speed to adjust the solder wave height which is associated with the pump motor.